1. Field of the Invention
This invention generally relates to a one-way clutch, and, in particular, to a ribbon spring for use in a sprag-synchronized one-way clutch.
2. Description of the Prior Art
FIG. 1 schematically illustrates the overall structure of a sprag-synchronized one-way clutch 1. As shown, the sprag-synchronized one-way clutch 1 is to be used as interposed between an inner ring 2 and an outer ring 3. When the inner and outer rings 2 and 3 rotate relative to each other in the directions opposite to the directions indicated by the arrows, their relative rotation is allowed; on the other hand, when the inner and outer rings 2 and 3 try to rotate relative to each other in the directions indicated by the arrows, the relative rotation is prevented from taking place. The sprag-synchronized one-way clutch 1 includes a pair of ring-shaped outer and inner retainers 4 and 5, and these retainers 4 and 5 are each formed with a plurality of sprag insertion windows or holes 4a and 5a, respectively. Also provided in the sprag-synchronized one-way clutch 1 are a plurality of sprags 6, each having a particular cross-sectional shape which is tapered in the middle. Each of the sprags 6 is inserted into the corresponding one of each of the sprag insertion holes 4a and 5a. Each of the sprags 6 has an outer cam surface 6a, which is defined by one or two or more radii of curvature, and an inner cam surface 6b.
A ribbon spring 7 is interposed between the inner and outer retainers 4 and 5, and a predetermined moment is normally given to each of the sprags 6 in a predetermined direction by the ribbon spring 7. The ribbon spring 7 is also formed with a plurality of sprag insertion holes 7a at a predetermined pitch, and each of the sprags 6 is inserted into the corresponding one of the sprag insertion holes 7a and maintained in position as supported at the middle. The ribbon spring 7 is generally elongated in shape and comprised of a thin plate of a desired material, preferably metal, such as stainless steel. After inserting the sprags 6 into all of the sprag insertion holes 7a, both ends of the ribbon spring 7 are put together to define a circular shape, and such a circularly shaped ribbon spring 7 with the sprags 6 is inserted between the outer and inner retainers 4 and 5. As shown in FIG. 1, when assembled, the ribbon spring 7 normally applies moment to each of the sprags 6 in the directions indicated by the arrows A, so that each of the sprags 6 is biased to be in contact with the inner and outer rings 2 and 3, to provide a coupling condition.
The structure of a typical prior art ribbon spring 7 is illustrated in FIGS. 2a and 2b. The ribbon spring 7 is typically comprised of an elongated thin plate of a metal, such as stainless steel, which is formed with a plurality of sprag insertion holes 7a, each generally in the shape of a letter "C" located at a predetermined pitch. A tab 7d is formed in each of the sprag insertion hole 7a as extending into the hole 7a from one side thereof. FIG. 2b is a side view of the ribbon spring 7 shown in FIG. 2a. The illustrated ribbon spring 7 includes a pair of elongated side sections 7b and 7b and a plurality of cross-bars or bridge sections 7c arranged at a predetermined pitch along the longitudinal direction of the ribbon spring as bridging between the pair of side sections 7b and 7b. And, each of the sprag insertion holes 7a is defined by the pair of side sections 7a and 7 a and a pair of adjacent bridge sections 7c and 7c.
The tab 7d is formed as a projection which projects into the corresponding sprag insertion hole 7a from one side of the corresponding bridge section 7c, so that the sprag insertion hole 7a is generally in the shape of a letter "C." The ribbon spring 7 serves to apply a predetermined moment to the sprag 6 in the direction indicated by the arrows in FIG. 1 so as to keep the sprag 6 biased to be in contact with the inner ring 2 and also with the outer ring 3. For this reason, it is necessary for the tab 7d to apply an appropriate pressure force to the associated sprag 6. In addition, a plurality of wavy or crooked portions 7e are formed in the ribbon spring 7 at a predetermined pitch along the longitudinal direction of the ribbon spring 7. In the illustrated embodiment, the crooked portions 7e are provided at the same pitch as that of the sprag insertion holes 7a and with a different phase. These wavy or crooked portions 7e are formed by locally bending portions of a thin plate at the same side in a predetermined shape, and each of the crooked portions 7e is formed in the pair of side sections 7b and 7b and the tab 7d in a transverse direction.
FIG. 3 illustrates the condition in which sprags 6 are inserted into the respective sprag insertion holes 7a of the ribbon spring 7 shown in FIGS. 2a and 2b and the both ends of the ribbon spring 7 are brought together to form a circular shape to be ready for assembling into the one-way clutch 1. In this manner, the prior art ribbon spring 7 extends straight in its longitudinal direction before assemblage, both ends of the ribbon spring 7 must be brought together to form the ribbon spring 7 in a circular or ring shape. In this case, however, when the ribbon spring 7 is formed in a circular shape, both end portions 7' and 7" tend to extend straight. Thus, when such a ribbon spring 7 is assembled into the one-way clutch 1, one end portion 7' of the ribbon spring 7 tends to push the other end portion 7" radially outwardly, so that the tip end of the end portion 7" located radially outwardly of the other end portion 7' comes to be pushed against the inner peripheral surface of the outer retainer 4. In addition, when the ribbon spring 7 is set in position as shown in FIG. 4, the end portion 7' of the ribbon spring 7 also pushes against the other end portion 7". Under this condition, the sprag 6' located at the meeting position between the end portions 7' and 7" of the ribbon spring 7 receives a moment which is different from the moment received by any other sprags 6. As a result, the contact condition between the sprag 6' and each of the inner and outer rings 2 and 3 differs from those of the other sprags 6, so that there is a lack of uniformity in clutch performance in the circumferential direction of the one-way clutch 1.
Furthermore, if the one-way clutch 1 is operated under this condition, every time when the clutch is coupled and decoupled with the end portion 7' being pressed against the end portion 7" and at the same time with the end portion 7" being pressed against the inner peripheral surface of the outer retainer 4, both of the end portions 7' and 7" are scraped against each other and with the outer retainer 4 for the end portion 7" in the circumferential direction repetitively. Thus, the end portions 7' and 7" gradually wear out. In this case, since the end portion 7" is scraped against the outer retainer 4 having a higher rigidity, it tends to be worn out more rapidly. The status of the ribbon spring 7 which has been worn in this manner is indicated in FIG. 4 by the dotted lines. As the end portions 7' and 7" of the ribbon spring 7 wear due to scraping as described above, they tend to be oriented more radially, which would increase the wear of the end portions 7' and 7", thereby causing the sprag 6' to be more unstable. Besides, there is a chance that the bottom of a wrinkle defined by the wavy or crooked portion 7e of the end portion 7' could wear out due to scraping with the outer peripheral surface of the inner retainer 5. Moreover, regarding the ribbon spring 7 itself, the stress at the end portion 7' or 7" becomes higher by the factor of approximately 1.5 to 2.5 as compared with the remaining portion of the ribbon spring 7, so that damage tends to occur especially in the vicinity of the end portion 7' or 7" of the ribbon spring 7.